Re: [APD] Atrazine

[David Aiken <daiken at iprimus_com.au>]

On 29/05/2006, at 7:26 AM, Vaughn Hopkins wrote:

> A carcinogen doesn't work by being poisonous.  It works by affecting
> the DNA of the cells.  About all that is changed by bigger doses,
> unless it is also poisonous, is how quickly the DNA damage is visible
> as cancer.  As I  posted before, only a small percentage of the
> chemicals that have been tested, at any level, are carcinogens.   
> Once a
> chemical is found to be carcinogenic, the wise thing is to assume  
> it is
> carcinogenic for all species and greatly limit or eliminate  
> exposure to
> it.  And, just as smoking does not mean that an individual will get
> cancer, but it does greatly increase the probability that he will, the
> same is true of other carcinogens to a lesser or greater degree.

This is a more reasonable statement of the situation than Vaughan's  
earlier post but still overly strong to my view.

"About all that is changed by bigger doses, unless it is also  
poisonous, is how quickly the DNA damage is visible as cancer. \"

I don't think this is absolutely true. Yes, carcinogens work by  
damaging cell structures. It's not true that all bigger doses do is  
change how quickly the DNA damage is visible as cancer. The period  
between when cell damage occurs and when the cancer becomes apparent  
is the latency period. For some cancers at least, the latency period  
appears to be relatively constant and unaffected by dose. I'm  
thinking of mesothelioma which has a very long latency period. Dose— 
the level of asbestos exposure—doesn't seem to affect the latency  
period but exposure to higher doses results in a higher incidence of  
the disease. In other words, exposure to asbestos dust does not mean  
that you will develop mesothelioma but exposure to greater quantities  
increases the likelihood that you will. The damage will take the same  
period to become apparent irrespective of the actual exposure received.

Exposure to UV in sunlight can cause a range of skin cancers and once  
again latency periods can be very long. Levels of exposure don't seem  
to affect latency all that much here either, at least for most  
"normal" exposures.

That's a different relationship to the one you're asserting. I think  
what you said is correct in some cases, but it definitely doesn't  
seem to be correct in all.


"Once a chemical is found to be carcinogenic, the wise thing is to  
assume it is carcinogenic for all species and greatly limit or  
eliminate exposure to it. "

The problem here is the exposure recommendation. Sunlight is  
carcinogenic, or at least the UV light in it is. Exposure to sunlight  
causes skin cancers including melanomas which can be lethal. We  
obviously don't want to eliminate exposure to sunlight because that  
creates a totally different set of problems, and we don't want to  
"greatly limit" it for the same reasons. We need a reasonable  
exposure to sunlight, and that includes some UV exposure. What we  
need to do is avoid excessive exposure, especially in the middle of  
the day when the sun is more directly overhead.

If there's one thing I've learned, it's that blanket recommendations  
like "greatly limit or eliminate exposure" can definitely be  
counterproductive.

There are some things which cause cancer which are not necessary to  
us, and for them we definitely are better off following your  
recommendation, and eliminating exposure if at all possible. We  
should only consider greatly reducing exposure in these cases if  
elimination is impossible.

There are other things which cause cancer but which are also  
essential to our health for some reason, and sunlight is a good  
example of this class of risks. Elimination is not an option, nor is  
"greatly limiting" which tends to mean "reduce as much as you can".  
We need to avoid excessive exposure in these cases, rather than  
greatly limiting exposure, and we need to understand just what levels  
of exposure are desirable and what levels are undesirable.

We need to know which cancer causing issue we're talking about before  
we start issuing exposure limit recommendations and there is no  
single recommendation that is equally valid in every case.

David Aiken

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